Broadband and High-Gain Metamaterial Microstrip Antenna

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Project Final Technical ReportSubmitted to the AFOSR/AOARD for Project

Case No. AOARD-094069 and Contract No. FA2386-09-1-4069

A Broadband and High-Gain Metamaterial

Microstrip Antenna

Principal Investigator: Prof. Joshua Le-Wei LiDepartment of Electrical & Computer Engineering

National University of Singapore

10 Kent Ridge Crescent, Singapore 119260

Email: [email protected]; Homepage: http://www.ece.nus.edu.sg/lwli

Program Manager: Dr Gregg Jessen

Asian Office of Aerospace Research and Development (AOARD)

The U.S. Air Force Research Laboratory (AFRL)

Projected dated on May 8, 2010


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1. REPORT DATE

13 JUL 2010

2. REPORT TYPE

Final

3. DATES COVERED

16-04-2009 to 10-05-2010

4. TITLE AND SUBTITLE

Broadband and High Gain Microstrip Antennas Using

Metamaterial/NIMs Concepts

5a. CONTRACT NUMBER

FA23860914069

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Joshua Le-Wei Li

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National University of Singapore,10 Kent Ridge Crescent,Singapore,SN,119260

8. PERFORMING ORGANIZATION

REPORT NUMBERN/A

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

Asian Office of Aerospace Research & Development, (AOARD), Unit

45002, APO, AP, 96338-5002

10. SPONSOR/MONITORS ACRONYM(S)

AOARD

11. SPONSOR/MONITORS REPORT

NUMBER(S)

AOARD-094069

12. DISTRIBUTION/AVAILABILITY STATEMENT

Approved for public release; distribution unlimited

13. SUPPLEMENTARY NOTES

14. ABSTRACT

A broad bandwidth and high gain rectangular patch antenna was specifically designed in this project using

planar-patterned metamaterial concepts. Based on an ordinary patch antenna, the antenna has isolated

triangle gaps and crossed strip-line gaps etched on the metal patch and ground plane, respectively.

Demonstrated to have left-handed characteristics, the patterned metal patch and finite ground plane form

a coupled capacitive-inductive circuit of negative index metamaterial. It is shown to have great impact on

the antenna performance enhancement in terms of the bandwidth significantly broadened from a few

hundred MHz to a few GHz, and also in terms of high efficiency, low loss and low voltage standing wave

ratio. Experimental data show a reasonably good agreement between the simulation and measured results.

This antenna has strong radiation in the horizontal direction for some specifical applications within the

entire band.

15. SUBJECT TERMS

Antennas, Metamaterials, Electromagnetic Simulation

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OFABSTRACT

Same as

Report (SAR)

18. NUMBER

OF PAGES

17

19a. NAME OF

RESPONSIBLE PERSONa. REPORT

unclassified

b. ABSTRACT

unclassified

c. THIS PAGE

unclassified


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Standard Form 298 (Rev. 8-98)Prescribed by ANSI Std Z39-18


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Contents

0.1 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

0.2 Resultant Direct and Indirect Publications . . . . . . . . . . . . . . . . . . . 2

0.3 Background Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

0.4 Proposed Structure of Single-Element Antenna and Metamaterial System . . 4

0.5 Metamaterial Microstrip Antenna Arrays . . . . . . . . . . . . . . . . . . . 8

0.6 New Physics of the Present Work and Future Applications . . . . . . . . . . 11

0.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

1


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A Broadband and High-Gain Metamaterial Microstrip Antenna 2

Abstract

A braod bandwidth and high gain rectangular patch antenna was specifically designed in this

paper using planar-patterned metamaterial concepts. Based on an ordinary patch antenna,

the antenna has isolated triangle gaps and crossed strip-line gaps etched on the metal patch

and ground plane, respectively. Demonstrated to have left-handed characteristics, the pat-

terned metal patch and finite ground plane form a coupled capacitive-inductive circuit of

negative index metamaterial. It is shown to have great impact on the antenna performance

enhancement in terms of the bandwidth significantly broadened from a few hundred MHz to a

few GHz, and also in terms of high efficiency, low loss and low voltage standing wave ratio.

Experimental data show a reasonably good agreement between the simulation and measured

results. This antenna has strong radiation in the horizontal direction for some specifical

applications within the entire band.

0.1 Acknowledgments

The Principal Investigator is grateful to the AFOSR/AOARD for their supported Projects:

AOARD-07-4024 and AOARD-09-4069. Also, he is especially grateful to Dr Gregg Jessen,

Program Manager at Asian Office of Aerospace Research and Development (AOARD), The

U.S. Air Force Research Laboratory (AFRL) for his strong support throughout the project(without which, the great progress of the project would not be possible), for his efficient

management of the project, and his technical discussions and suggestions provided along the

way of the project.

The PI is also grateful to the Defence Science and Technology Agency of Singapore and

the National University of Singapore for the partial financial support in terms of Defence

Innovative Research Program with Project No: DSTA-NUS-DIRP/2007/02. The $600,000

grant support provides a good platform, especially the code development part, to support

this project.

0.2 Resultant Direct and Indirect Publications

Resulted in from the supported Projects: AOARD-07-4024 and AOARD-09-4069, the fol-

lowing research papers were published:

Le-Wei Li, Ya-Nan Li, Tat-Soon, Yeo, Juan R. Mosig and Olivier J.F. Martin, A

Broadband and High-gain Metamaterial Microstrip Antenna, Applied Physics Letters,vol.96, no. 6, 164101, April 2010


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Li Hu, Le-Wei Li, and Raj Mittra, Electromagnetic Scattering by Finite Periodic

Arrays Using CBFM/AIM, IEEE Transactions on Antennas and Propagation, vol.

58, to appear, 2010

Wei-Jiang Zhao, Le-Wei Li, and Li Hu, Efficient Current-Based Hybrid Analysis of

Wire Antennas Mounted on a Large Realistic Aircraft, IEEE Transactions on Anten-

nas and Propagation, vol. 58, no. 8, August 2010

Wei-Jiang Zhao, Le-Wei Li, and Ke Xiao, Analysis of Electromagnetic Scattering and

Radiation from Finite Microstrip Structures Using an EFIE-PMCHWT Formulation,

IEEE Transactions on Antennas and Propagation, vol. 58, no. 7, July 2010

There are a few other papers completed and submitted to journals for publications.

They are still under-review and thus are not listed herewith.

0.3 Background Introduction

A microstrip patch antenna [1, 2] represents one of the most commonly utilized printed

antennas in practice. It enjoys its advantages of low profile, simple structure, low cost,

and omnidirectional radiation patterns [1, 2]. A narrow bandwidth is, however, the main

drawback of the microstrip patch antennas. Some approaches have been therefore developed

for bandwidth enhancement [36]. Among those common ones, one is to increase the height

of the dielectric substrate while the other is to decrease the substrate dielectric constant.

Certainly, the latter will induce the matching circuits to be impractical due to excessively

wide lines designed.

Usually microstrip patch antennas have the strongest radiation in the vertical direction

(while the patch is placed horizontally). At the mean time, the radiation along the plane of

patch usually causes unwanted radiated power (due to surface waves such as lateral waves and

leaky waves) and thus causes coupling among array elements so as to reduce the efficiency [4].

Dielectric polarization currents were confirmed as the physical sources of this unwantedradiation. In practical specific applications, it is, however, desirable to have the radiation

along the horizontal plane.

Since the artificial left-handed materials (LHMs) or metamaterials were proposed, theo-

retically characterized, and experimentally realized [713], scientists and engineers have tried

various ways to bring these special material characteristics into practical applications. The

metamaterials have been successfully applied in optical frequency band for optical imag-

ing [1417]. Although it is easier to realize metamaterials in microwave frequency region

for negative refractions, there was still little progress toward practical applications [18]. Atmicrowave frequencies, potential applications include primarily (a) substrate materials for


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antenna and microwave component designs and fabrications, and (b) absorbing materials for

engineering and radar applications. For example, split ring resonators (SRRs) [11,1921] and

some other planar structures [2224] were applied in some antenna fabrications to minimize

the size and enhance the radiation. Also in some other designs, artificial magnetic materi-als [10, 25] with stacks of SRRs under patch antenna were proposed and it was found that

the resonant frequency of the original patch antenna can be significantly reduced [26]. There

are, however, still primarily fundamental issues at microwave frequencies: narrow bandwidth

(when both negative permittivity and negative permeability merge in the same band) and

high loss (due to the ohmic loss and radiation loss of inclusion elements), and this drawback

becomes especially serious when the SRR- and other inclusion-types of metamaterials are

used as substrate of the patch antenna.

The objective of this paper is thus to enhance, in a completely different approach, thebandwidth and gain of a conventional patch antenna by applying the planar metamate-

rial patterned structures directly on the upper patch and bottom ground of the dielectric

substrate, so the patch antenna can have an excellent performance.

0.4 Proposed Structure of Single-Element Antenna and

Metamaterial System

A conventional microstrip patch antenna is usually mounted on a substrate and backed

by a conducting ground plane. In the present investigation, as shown in Fig. 1, a planar

left-handed material pattern on the rectangular patch antenna mounted on the substrate is

designed to enhance its horizontal radiation as well as to broaden its working bandwidth via

its coupling with the conducting ground backed to the substrate and patterned in a different

way. On the upper patch, the periodic gaps are designed in the form of isolated micro-

triangles; while on the bottom ground plane, as shown in Fig. 1(a), periodically distributed

cross strip-line gaps are designed. To maintain the transmission consistency of input energy,

the metal in and around the feed-line area is, however, not etched. The prototype andthe dimension of the two patterned planes of the proposed antenna are shown in Fig. 1.

The left-handed characteristics of these patterns were already demonstrated in [27] and thus

will not be further discussed here. We have also optimized the original patterns slightly

to achieve a better performance of the antenna. Physically, the upper patch and bottom

plane are coupled to form a capacitive-inductive (C-L) equivalent circuit and thus can induce

backward wave which travels along the plane of patch. In this connection, the radiation along

the patch direction is significantly enhanced.

A conventional antenna of the same size is used as a reference for comparison and both

the conventional rectangular patch and the proposed patch antenna are fed by an off-centered


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Figure 1: (Color online) Configuration in 3D of the proposed patch antenna.

microstrip line. The substrate used here is Duroid with a relative permittivity ofr

= 2.2,

and its thickness is 31 mil. The area of the upper patch mounted is 12 16 mm2. Two

different widths g of the gaps (i.e., 0.3 mm and 0.4 mm) on the ground plane are assumed.

Therefore, different inductance components of the LHM characteristics can be obtained so as

to control the resonance frequency and also the working bandwidth of the designed antenna.

It is realized that the patterned ground plane produces negative permeability in the first

frequency bandwidth while the patterned patch antenna produces negative permittivity in

the second frequency bandwidth. (a) If only the patterned ground plane or the patterned

patch antenna is utilized, then only single negative medium effect can be observed and it

produces the stop-band. Therefore, both the patterned ground plane and the patterned

patch antenna are used. (b) If the double negative permittivity and permeability fall within

different bandwidths and there is no overlap, then it still produces the stop-band. (c) If the

double negative permittivity and permeability fall within the same bandwidth or there is big

overlap between the two bandwidths, then it produces the pass-band so that the energy can

be radiated by the antenna. Of course, the impedance matching should be always considered.

The prototypes LHM patch antenna was numerically simulated/designed, physically

fabricated, practically measured and comparatively studied with theoretical results. Tooptimize it for various parameters, a full wave finite element method simulator was used.

The computed S11 values of the proposed antenna and the reference patch antenna are

obtained and shown in Fig. 2(a). As seen, the working bandwidth of the conventional patch

antenna is 200 MHz (between 7.1 GHz and 7.3 GHz), which is typically very narrow as

expected but serves as a benchmark for the improved designs. The proposed antenna is

designed to have the 0.4 mm gap at the bottom, and the 10 dB bandwidth (which is

standardly defined for engineering applications) falls within 5.3 GHz and 8.5 GHz (which is

3.2 GHz in bandwidth, and is 16 times wider than the conventional antenna). When the gap

at the bottom becomes 0.3 mm, the 10 dB bandwidth turns within 5.7 GHz and 8.6 GHz(which is 2.9 GHz in bandwidth, and is 14.5 times wider than the conventional antenna).


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Figure 2: (Color online) S-parameter values of the proposed antenna with different gap

widths. (a) Computed S11 values for metamaterial antenna and conventioal patch. (b)Measured and computed S11 values for metamaterial antennas.

To verify the accuracy of designs, two proposed antennas with different gap widths (one

of which is shown in Fig. 3) are then fabricated and measured. Experimental results of S11values are compared with numerically predicted results in Fig. 2(b). The general variational

trace of the experimental results follows closely to that of the simulated S11 values. From the

10-dB level line, it is seen that a reasonably good agreement between the design working

bandwidth and the measured bandwidth is found. In fact, the fabricated antennas have even

wider bandwidths than those of modeled antennas.

Figure 3: (Color online) (a) Top and (b) bottom views of a fabricated patch antenna.

Antenna gains are measured within the entire frequency band as shown in Fig. 4(a).


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The antenna gain is generally above 4 dB with the peak of 7.2 dB. For a patch antenna,

this has been a very high as compared with that of a standard one. The voltage standing

Figure 4: (Color online) (a) Measured gain and (b) simulated VSWR of proposed antenna

with a gap of 0.4 mm. Shaded area shows the working band.

wave ratio (VSWR) is defined as (1+ ||)/(1||) where denotes the reflection coefficient.

The VSWR value serves as a good measure to check if the system is working efficiently.

In the present work, the simulated VSWR value is well below 2 (which serves as a good

reference value for most of the engineering applications) within the frequency band as shown

in Fig. 4(b).

Due to the left-handed transmission characteristics, the wave propagation along the

patch induces the strongest radiation in horizontal direction instead of the vertical direction

of the conventional patch antenna. To further confirm this, two frequencies, i.e., 6.66 GHz

and 7.77 GHz both in the working bandwidth, are randomly chosen to characterize radiation

of the antenna. According to the computed results, the 3D radiation patterns at these

two frequencies are shown in Fig. 5. It is apparent that energy radiates to the horizontal

directions.

To further verify the results, the measured co-polarization and cross-polarization radia-tion patterns are plotted in 2D in Fig. 6, respectively. The gain is found to be able to reach

as high as 7.14 dB, which is quite desirable for a single patch and it would never happen if

a conventional patch antenna is designed. Also seen from the 2D patterns at both of these

randomly picked frequencies, the radiated energy is mainly focused in the x-direction in the

case of the co-polarization. In the case of the cross-polarization, the radiation level is well

suppressed except at around 210 of the -direction in the xy-plane. So we may also take

a good advantage of this special characteristic to transmit two quadrature signals using the

antenna as a directional one for beam control.


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Figure 5: (Color online) Computed 3-D radiation patterns at (a) 6.66 GHz and (b) 7.77

GHz, respectively.

0.5 Metamaterial Microstrip Antenna Arrays

After we successfully designed the single patch element, we then move to design the meta-

material microstrip array. The 4-element microstrip array is of the size (88 mm 30 mm)

and fed by a microstrip line network, as shown in Fig. 7 [28,29].

Table 1: Computed antenna gains versus number of elements and operating frequency.

4 ele. 6 ele. 8 ele. 10 ele.

6.70 G 8.9418 9.8778 10.629 11.250 11.776 12.226 12.615

7.70 G 10.007 10.932 11.670 12.276 12.783 13.210 13.574


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Figure 6: (Color online) Measured radiation patterns of co-polarization (solid line) and cross-

polarization (dotted line) (a) at 6.66 GHz in the xy-plane; (b) at 7.77 GHz in the xy-plane;

(c) at 6.66 GHz in the yz-plane; and (d) at 7.77 GHz and in the yz-plane, respectively.


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20mm

y

short to GND

x

input

88mm

79.1

mm

30mm

re served for isolatio n resisto r

Figure 7: Microstrip antenna array geometry.

Figure 8: Computed return loss of the 4-element array.


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Figure 9: Computed antenna gains versus operating frequency.

of surface waves can be reduced by introducing insolation technique between elements, such

as adding some band gap structures [30,31].

Fig. 9 shows the computed gain of the proposed array and the original array, the original

array also have high gain, but when the frequency is below about 7.1 GHz or higher than

about 8.9 GHz, radiation pattern changed a lot, for some leaky modes and high modes are

excited, and the maximum radiation direction of leaky modes changed with frequency [32,33],

so as the radiation pattern changed. The radiation pattern of the proposed array remains

good in the frequency range of 5.5 GHz to 9.2 GHz as shown in Fig. 10.

In frequency of 8.2 GHz, the and components of radiation pattern in = 900 plane

of the proposed array are plotted in Fig. 11. Similar to the single antenna element, the main

radiation direction in = 900 plane (in far-field) is direction. It is found that the antenna

array patterns are consistently strong in the horizontal plane, as expected for the entireworking frequency bandwidth. The radiation pattern remains radiation in patch direction

in a broadband frequency.

0.6 New Physics of the Present Work and Future Ap-

plications

In this project, we have proposed and implemented a new idea of using the negative permit-tivity and negative permeability of the circuitry in the microstrip antennas and microstrip


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(a) 5.5 GHz (b) 6.0 GHz

(c) 7.0 GHz (d) 8.0 GHz

(e) 9.0 GHz (f) 9.2 GHz

Figure 10: Computed 3-D radiation patterns of the antenna array with 4 elements.

antenna arrays into their practical applications. The new antennas, now known as the meta-

material microstrip antennas and arrays or metamaterial flower antennas and arrays, are


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Figure 11: Radiation components in = 900 plane of 8.2 GHz.

found to have much broader frequency bandwidths and also high gains.

There are a few points which need to be addressed clearly. In the present antenna design,

the primary idea used is the metamaterial concepts where the antenna substrate material

itself is still a conventional one.

From this point of view, the design concept is very different from those designs where

the electromagnetic bandgap (EBG) structures or materials are employed. The EBG

structures were utilized in the design and fabrication of microstrip antennas and arrays

so that the surface waves propagating along the microstrip patch(es) can be suppressed,

and therefore the total radiation efficiency can be enhanced significantly toward the

direction normal to the patches. The present design in fact fully utilizes the propertiesof the surface waves so that the radiation can be enhanced towards the same direction

of the surface waves.

Secondly, the design concept is also very different from the frequency selective surface

(FSS) idea. The FSS periodic structures are of very strictly designed patterns so

that the responses of the FSS structures can meet the specific requirements by the

system components. In the metamaterial antenna design, the periodic patterns are

also integrated inside the designs, but it does not require the pure periodicity of the

patterns. Also the purpose of employing periodic patterns in the metamaterial designs

are entirely different from the FSS structures.


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In short, a new concept of negative index materials (NIM) was used in the design of the

metamaterial antennas. It is different from either the frequency selective surface concept

or the electromagnetic bandgap concept. The performance of the metamaterials designed is

found better than those antennas designed using the EBG concept, while the FSS conceptwas usually applied to make optimized radar cross section designs of air or ocean vehicles or

objects.

As the metamaterial flower antennas are proved to have high performance, therefore,

it is expected that the concept can be applied to the future practical antenna designs. So,

the future work will be applications of the optimumly designed metamaterial antennas for

practical applications, especially the defence applications. The next work might be the

demonstration project to be proposed for the defense component designs.

0.7 Conclusions

A novel broad bandwidth microstrip antenna and also an array of this kind are proposed

and designed using the metamaterial concept via the pattern-etched upper patches and bot-

tom ground plane which form a C-L equivalent circuit of negative index for enhancing the

antenna array performance. Some simulated and measured results are obtained and the

working frequency bandwidth of the rectangular microstrip antenna and array are signifi-cantly broadened from about 200 MHz to about 3 GHz (at about 8-15 times). Also, this

new patch antenna designed using the metamaterial concept has very high efficiency of above

98% according to simulation, very low loss (or high gain according to measurements) and

low voltage standing wave ratio.

Practical experiments demonstrate a reasonably good agreement between simulation and

measurement results of the S-parameters and the antenna patterns. Both theoretically and

experimentally, it is depicted that the antenna could radiate toward the horizontal directions

within the entire working frequency band, while the vertical radiation is well suppressed

which could lead many practical applications in wireless communication subsystems.


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Broadband and High-Gain Metamaterial Microstrip Antenna

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